1. Field of the Invention
The present invention relates to an electronic device that transmits moving images including video information and audio information which are synchronized with each other.
2. Description of the Related Art
The Sony Philips digital interface format (SPDIF) is a widely used audio data transmission format. For example, the SPDIF is even employed as a part of high definition multimedia interface (HDMI).
SPDIF supports both an uncompressed audio data transmission format (IEC60958) and a compressed audio data transmission format (IEC61973). Since SPDIF allows transmission in such different audio data formats, the SPDIF packets include meta information indicating the transmission audio data format in a payload section that stores the actual data. This meta information is utilized as an audio data format identifier.
This meta information is stored by splitting it into 192 packets per bit. Thus, the meta information can be restored by collecting these 192 packets (FIG. 2A). The 192 packets required to restore the meta information are called an “audio block”. During restoration, the meta information cannot be switched. However, there is no stipulation defining the synchronization relationship between the meta information and the payload section. Therefore, when switching the audio data format, a mismatch can occur between the meta information and the payload section. For example, when switching to audio code number 3 (Dolby Digital, “AC3”) while the meta information is in linear pulse-code modulation (LPCM) format in an audio block, the receiver mistakes the AC3 data for LPCM data. Consequently, an abnormal noise maybe output for this audio block (FIG. 2B).
Japanese Patent Application Laid-Open No. 2008-107397 discusses a realistic way of switching audio data formats. In the technique discussed in Japanese Patent Application Laid-Open No. 2008-107397, a different signal line to the digital audio signal line (SPDIF) in HDMI is used to notify the receiver of the switching period for the audio data format. The receiver, which has been notified of the switching period, plays back a mute sound during that period without using the transmitted digital audio data. Consequently, the occurrence of an abnormal noise is suppressed.
Since the technique discussed in Japanese Patent Application Laid-Open No. 2008-107397 is based on the using of a special signal, for a general-purpose device, the occurrence of abnormal noise cannot be suppressed. Further, for a transmission method other than HDMI, for example a method in which the video signal is transmitted via a D terminal and the audio signal is transmitted by SPDIF, the technique discussed in Japanese Patent Application Laid-Open No. 2008-107397 can not be applied.
In addition, in the technique discussed in Japanese Patent Application Laid-Open No. 2008-107397, the signal notifying the switching period of the audio data format is asynchronous to the audio data. Consequently, the notification of the switching period has to be performed with a sufficient margin with respect to the switching timing of the audio data format, which means that the mute period is longer.